C2H2 Lewis structure

C₂H₂ (acetylene or ethyne) has two carbon atoms and two hydrogen atoms.

In the C₂H₂ Lewis structure, there is a triple bond between the two carbon atoms, and each carbon is attached with one hydrogen atom, and none of the atoms has a lone pair.

Alternative method: Lewis structure of C₂H₂

Contents

Rough sketch

First, determine the total number of valence electrons

In the periodic table, carbon lies in group 14, and hydrogen lies in group 1.

Hence, carbon has four valence electrons and hydrogen has one valence electron.

Since C₂H₂ has two carbon atoms and two hydrogen atoms, so…

Valence electrons of two carbon atoms = 4 × 2 = 8
Valence electrons of two hydrogen atoms = 1 × 2 = 2

And the total valence electrons = 8 + 2 = 10

Learn how to find: Carbon valence electrons and Hydrogen valence electrons

Second, find the total electron pairs

We have a total of 10 valence electrons. And when we divide this value by two, we get the value of total electron pairs.

Total electron pairs = total valence electrons ÷ 2

So the total electron pairs = 10 ÷ 2 = 5

Third, determine the central atom

Here hydrogen can not be the central atom. Because the central atom is bonded with at least two other atoms, and hydrogen has only one electron in its last shell, so it can not make more than one bond.

Now there are only two atoms remaining and both atoms are carbon, so we can assume any one as the central atom.

Let’s assume that the central atom is right carbon.

Therefore, place carbons in the center and hydrogens on either side.

And finally, draw the rough sketch

C2H2 Lewis Structure (Step 1) — Rough sketch of C₂H₂ Lewis structure | Image: Learnool

Lone pair

Here, we have a total of 5 electron pairs. And three bonds are already marked. So we have to only mark the remaining two electron pairs as lone pairs on the sketch.

Also remember that carbon is a period 2 element, so it can not keep more than 8 electrons in its last shell. And hydrogen is a period 1 element, so it can not keep more than 2 electrons in its last shell.

Always start to mark the lone pairs from outside atoms. Here, the outside atoms are hydrogens and left carbon. But no need to mark on hydrogen, because each hydrogen has already two electrons.

So for left carbon, there are two lone pairs, and for right carbon, there is zero lone pair because all two electron pairs are over.

Mark the lone pairs on the sketch as follows:

C2H2 Lewis Structure (Step 2) — Lone pairs marked on C₂H₂ Lewis structure | Image: Learnool

Formal charge

Use the following formula to calculate the formal charges on atoms:

Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons

For left carbon atom, formal charge = 4 – 4 – ½ (4) = -2

For right carbon atom, formal charge = 4 – 0 – ½ (4) = +2

For each hydrogen atom, formal charge = 1 – 0 – ½ (2) = 0

← Scroll horizontally to view all columns →

Atom	Valence electrons	Non-bonding electrons	Bonding electrons	Formal charge
Carbon (left)	4	4	4	-2
Carbon (right)	4	0	4	+2
Hydrogen (all)	1	0	2	0

Here, both carbon atoms have charges, so mark them on the sketch as follows:

C2H2 Lewis Structure (Step 3) — Formal charges marked on C₂H₂ Lewis structure | Image: Learnool

The above structure is not a stable Lewis structure because both carbon atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.

Convert a lone pair of the left carbon atom to make a new C — C bond with the right carbon atom as follows:

C2H2 Lewis Structure (Step 4) — Lone pair of left carbon is converted, but still there are charges | Image: Learnool

Since there are charges on both carbon atoms, again convert a lone pair of the left carbon atom to make a new C — C bond with the right carbon atom as follows:

C2H2 Lewis Structure (Step 5) — Lone pair of left carbon is converted again, and got the stable Lewis structure of C₂H₂ | Image: Learnool

Final structure

The final structure of C₂H₂ comprises two central carbon atoms connected to each other by a triple covalent bond, with each carbon also bonded to one hydrogen atom through a single bond. Within this layout, both carbon atoms satisfy the octet rule by forming four total bonds. Each hydrogen atom reaches its stable duet state through its single shared electron pair. This setup is the most stable because it results in formal charges of zero for all atoms involved, representing the most energetically favorable state for the molecule. Thus, this specific electronic distribution serves as the definitive and most accurate Lewis representation of acetylene.

Next: CH₂O Lewis structure

External video

How to Draw the Lewis Dot Structure for C2H2: Acetylene (Ethyne) – YouTube • Wayne Breslyn

External links

Deep

Learnool.com was founded by Deep Rana, who is a mechanical engineer by profession and a blogger by passion. He has a good conceptual knowledge on different educational topics and he provides the same on this website. He loves to learn something new everyday and believes that the best utilization of free time is developing a new skill.